Mainframe computer systems such as the IBM S/390 exchange data between input/output devices and main storage by I/O operations collectively known as the channel subsystem. The I/O devices and their control units attach to the channel subsystem. The channel subsystem is a combination of hardware and software which directs the flow of information between the control units of the I/O devices and main storage to relieve the computer Central Processing Unit (CPU) of the task of communicating with the I/O devices. Input and output processing can therefore take place concurrently with normal data processing in the CPU. I/O processing includes path management, testing paths for availability, choosing an available channel path and initiating and terminating the execution of I/O operations with the I/O device. A control unit is a mechanism for converting the characteristics of a particular I/O device to the standard forms used by the channel subsystem.
In accordance with prior art input-output operations, a sub-channel is provided for and dedicated to each I/O device accessible to the channel subsystem. Such sub-channels are logical rather than physical transmission paths and are defined in terms of the specific requirements of the associated I/O device. The number of such sub-channels is limited only by address sizes recognized by the computer system. Thus, in accordance with prior art input-output operations, I/O devices are attached to the channel subsystem by means of logical sub-channels derived on the physical transmission media (called channel paths) and activated when I/O operations are required. Logical sub-channels are defined as transmission facilities for varying sized data packet lengths ranging from zero to 32 kilobytes in four kilobyte steps. I/O control units are attached to the channel subsystem by one or more of such logical sub-channels and an I/O device can be served by one or more control units. Most I/O devices are designed to transfer data at only one specific rate and must be served by a logical sub-channel equaling or exceeding this device rate. Although the physical transmission media can be operated in half or full duplex mode, logical sub-channels have, for simplicity, nevertheless been defined for both directions of transmission to support both input and output. The prior art provided only for logical transmission sub-channels which are equal, balanced and symmetrical for both directions of transmission. Devices were assigned unique sub-channels during their installation and communication over that sub-channel conformed to the communication protocols defined for that type of I/O device. Note that all sub-channels in the prior art are pre-defined for a particular I/O destination, are balanced, and are merely activated when the system requests I/O. A great deal of computer input and output programming exists using the protocols described above.
The number of logical sub-channels in such a system is independent of the number of channel paths; the same I/O device can be accessed by a number of different channel paths, represented by a single logical sub-channel. The logical sub-channel maintains the main store addresses where the data is to be read or written, a count of the data transferred, the identity of the destination and storage for status information about the connected I/O device. This status information is accessible by programmed processes. Historically, logical sub-channels were derived over multi-conductor cables with bits transferred in parallel over different copper conductors (byte multiplex mode). Such cables were limited in length (30-50 meters) and bandwidth (1.5-4.5 Mb/s) and much existing I/O programming is adapted to these limitations. This older type of transmission medium is know as the parallel I/O interface and is further described in "IBM System/360 and System/370 I/O Interface Channel to Control Unit OEMI (Original Equipment Manufacturer Interface)," IBM System Library document GA22-6974, available from the IBM Corporation.
More recently, new optical fiber media have become available for interconnecting mainframes with I/O devices. This is a serial I/O interface for the channel subsystem, has a bandwidth of at least 20 Mbytes/s and can be extended by many kilometers (50-60 kilometers) by RCE repeaters (Remote Channel Extenders). One type of serial interface channel path is known as an ESCON (Enterprise System Connectivity) channel and is further described in "IBM Enterprise Systems Architecture/390 ESCON I/O Interface," IBM System Library Document SA22-7202, also available from the IBM Corporation. The ESCON serial I/O interface utilizes bursts of serial bit packets (burst mode) and presents problems to the standard I/O sub-channel system due to the higher capacity bandwidth, the delay latency inherent in the longer distances covered by the medium, system integrity (assuring matching logical assignments at both ends of the medium), and synchronization problems arising from these changing physical parameters.
One basic problem, then, in the channel subsystem is the inflexibility in defining and activating sub-channels. In modern applications such as Systems Network Architecture (SNA) and Advanced Peer-to-Peer Networks (APPN), greater flexibility is desirable, adapting to both byte multiplex and burst mode transmission paths in such a fashion as to be transparent to the existing user applications designed when only the byte multiplex mode was available. In addition, the possibility of breaking long data streams into segments for transmission over different logical sub-channels which might be implemented over different channel paths increases the sequencing problem, particularly in the face of channel path failures. Moreover, some degree of dynamic control over the parameters of I/O sub-channels is useful, and sometimes necessary, in advanced systems such as SNA and APPN. For example, users may need to negotiate such things as receiver buffer size or maximum transmission link capacity available. Finally, many modern devices and systems do not require balanced transmission channels having the same capacity in both directions since many multimedia applications involve minor control activities to control very wide broadband multimedia signals.
More particularly, it has been common in the past to provide for I/O devices comprising simple devices such as printers, magnetic tape units, magnetic disk units, terminals and sub-channels of other data processing systems. Today, however, such computers communicate with devices at great geographical distances connected by long transmission systems which might include local or wide area networks. Such interconnected systems have different requirements than simpler I/O devices. In particular, as noted above, services such as multimedia distribution require heavily unbalanced transmission capabilities for the two opposite directions of transmission. In addition, it is not always possible to have predetermined knowledge of the facilities available at the other end of a long I/O channel, and provision is not available in the prior art I/O supervisors to communicate the necessary parameters between the two ends of the sub-channel. Some form of system security is also desirable to ensure that the two ends of the sub-channels have the proper permissions for interconnection. Finally, the possibility of breaking large data blocks into segments for re-transmission in the face of channel path failures raises the possibility of segment and sub-segment sequencing problems not present in simple single sub-channel systems.
Today's channel attached computer configurations present a radically different environment from the traditional channel environment. Previously, it was possible to assume geographic proximity which implied a degree of local control over important aspects of the channel system design. Today's more vibrant and diversified channel environment gives rise to a number of system design problems. For example, channel security was easy to ensure when distances were limited to units expressed in terms of meters and there was little opportunity for an invasion of the system by a hostile system. In contrast, today's channel distances are measured in units of kilometers. Channel extender technology expands that distance to world-wide connectivity. In addition, earlier systems provided a degree of security by the use of pre-defined, static definitions. The flexibility of dynamic definitions also gives rise to increased exposure to invasion by a hostile system.
Another problem with prior art channel systems is the implied local knowledge of the buffering capacity of a sub-channel partner. In such prior art systems, any given implementation specific to a sub-channel partner assumed values for the partner's capacity. If in error, these assumptions lead to significant system inefficiencies. Similarly, statically pre-defined directions of transmission, either half duplex or simplex, is often unsuitable for particular data transfers. Dynamically defined and changeable directions of transmission would better suit today's more flexible data transfer requirements. Finally, user applications are normally designed with a particular device or device type in mind and the higher user level protocols are adapted to this particular device or device type. This requires the implementation of an interface for each user application to adapt the user protocols to the channel subsystem. A single methodology could replace these existing interfaces by providing a single set of system interfaces.
Prior art channel interfaces provide for only balanced bandwidth allocation interfaces. Such balanced allocation interfaces require that the transmit, or write, bandwidth is equal to the receive, or read, bandwidth. While this approach is suitable and, indeed, desirable, in a prior art transactions or low performance batch environments due to the essentially balanced data flow, today's client/server environments, by their very nature, dictate heavily unbalanced bandwidth interfaces. Similarly, new Asynchronous Transfer Mode (ATM) services on Wide Area Networks (WANs) likewise provide for unbalanced network interfaces.